What version of Vbox are you using and what host OS are you running on?

You don't normally need to activate any of the versions of guest additions presented in Driver Manager if you've installed them already via the devices > insert guest additions CD image method in VBox.

The Driver Manager versions are from the Ubuntu repo and are matched to the vbox version available in the repo. They may not work well or at all if they don't match the Vbox version running on the host.

I was actually just testing what happens if I install them from Driver Manager on Vbox 5.2.18 on a Cinnamon 19 host with a Mate 19 guest with the guest additions already installed - the install hung - restarted driver manager and now I appear to have lost net access in the guest.. So yeah best to avoid the Driver Manager versions...